This application is based on Japanese Patent Application Nos. 11-224534 (1999) filed Aug. 6, 1999, and 11-264493 (1999) filed Sep. 17, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical amplifier applicable to an optical fiber communication system and the like, and utilizing an active optical fiber as its gain medium, and to a Raman amplifier capable of improving its gain and pumping efficiency and to an optical fiber communication system using the Raman amplifier.
2. Description of the Related Art
As conventional examples of such optical amplifiers, there are a first configuration as shown in FIG. 21 (see, P. F. Wysocki, et al., IEEE Photon. Technol. Lett., vol.9, pp.1343-1345, 1997), and a second configuration as shown in FIG. 22 (see, E. Desurvire, Erbium-Doped Fiber Amplifiers, John Wiley and Sons, Inc., Section 6.2, 1994). In the optical amplifier of the first conventional configuration as shown in FIG. 21, signal light incident on a first optical isolator 41 is combined by a first optical combiner 42 with pump light fed from a first pumping light source 43, and is incident on a first erbium-doped fiber (called EDF from now on) 44. The EDF 44 amplifies the signal light using the pump light, and the amplified signal light passes through a second optical isolator 45 to be incident on a partially reflective gain equalizer 46. The signal light emerging from the partially reflective gain equalizer 46 passes through a third optical isolator 47, and is combined by a second optical combiner 48 with pump light fed from a second pumping light source 49 to be incident on a second EDF 51. The second EDF 51 amplifies it using the pump light, and the amplified light signal is emitted through a fourth optical isolator 52. In FIG. 21, the symbol xe2x80x9caxe2x80x9d designates a fusion splice.
In the second conventional optical amplifier as shown in FIG. 22, signal light is incident on a first port of an optical circulator 61 and emitted from a second port so that it is combined by an optical combiner 63 with pump light fed from a pumping light source 64, and the coupled signal light and pump light are incident on an erbium-doped fiber (EDF) 65, in which the signal light is pumped and amplified by the pump light. The amplified signal light is incident on a wavelength independent reflector 66 which reflects off the signal light to be incident on the EDF 65 from the opposite direction to the first incident direction. Then, the signal light passes through the EDF 65 and is incident on the second port of the optical circulator 61 via the optical combiner 63. The signal light is supplied from the third port of the optical circulator 61 to a gain equalizer 62 to be output therefrom.
The optical amplifiers of the first and second conventional configurations as shown in FIGS. 21 and 22 use rare-earth doped fibers such as the erbium-doped fibers (EDF) 44, 51 and 65 as a gain medium. The rare-earth doped fibers include besides the erbium-doped fiber a praseodymium-doped fiber, thulium-doped fiber depending on the type of the rare-earth material.
The partially reflective gain equalizer 46 is placed in an intermediate position to carry out gain equalization with maintaining the noise figure and optical output of the optical amplifier at good values. The partially reflective gain equalizer 46 consists of a Bragg fiber grading that equalizes the gain spectrum by reflecting part of the signal light in the direction opposite to its propagation direction to give a transmission loss, thereby providing the transmission loss with wavelength dependence.
The partially reflective gain equalizer 46 is interposed between the optical isolators 45 and 47 to prevent the signal light or spontaneously emitted light, which is reflected or amplified by the partially reflective gain equalizer 46, from returning to the first or second EDF 44 or 51, thereby preventing noise because of their return. The two optical isolators 41 and 52 at the two ends are installed to prevent the optical amplifier from becoming instable because of residual reflection light from outside.
FIGS. 23A and 23B illustrate gain spectra of the optical amplifier of FIG. 21, and a loss spectrum of the gain equalizer 46, respectively. FIG. 23A comparatively illustrates two spectra when the gain equalizer is used and not used, which illustrates that the gain equalization is carried out between the wavelengths xcex1 and xcex2.
In the second configuration of the optical amplifier as shown in FIG. 22, the pumping efficiency is improved by using the optical circulator 61 and wavelength independent reflector 66. Since the signal light reflected by the wavelength independent reflector 66 passes through the erbium-doped fiber (EDF) 65 twice, the gain (in terms of dB) is twice that of one passage. In addition, since the pump light is also reflected by the wavelength independent reflector 66, the pump light in the EDF is enhanced. FIGS. 24A and 24B illustrate a reflectivity spectrum and a loss spectrum of the wavelength independent reflector 66, respectively. The reflectivity is set high (close to 100%), and the loss is set low. The signal light passing through the EDF 65 twice is guided to the signal light output port by the optical circulator 61, and is output through the gain equalizer 62. The gain spectrum of the amplifier and the loss spectrum of the gain equalizer 62 are the same as those of FIGS. 23A and 23B. The gain equalizer 62 used by the second configuration can be either partially reflective or antireflective. The antireflective gain equalizer includes a Mach-Zehnder optical filter or long period fiber grating.
FIGS. 25A and 25B each show part of the erbium-doped fiber 65 and wavelength independent reflector 66 used in the second configuration: FIG. 25A shows a silicate erbium-doped fiber 65a, whereas FIG. 25B shows a non-silicate erbium-doped fiber 65b. As for the silicate erbium-doped fiber (EDF) 65a, it usually undergoes fusion splicing (denoted by the symbol xe2x80x9caxe2x80x9d) with adjacent pigtail fibers 67. Generally, the core diameter of the EDF is considerably smaller than that of the pigtail fibers 67, and hence the fusion splicing of the two requires much expense and time. On the other hand, as for the non-silicate erbium-doped fiber (EDF) 65b, since the fusion splicing between the EDF and the pigtail fibers 67 cannot be achieved, high NA (high numerical aperture) silicate fibers 68 and the EDF are butted and bonded at angled polished surfaces (denoted by the symbol xe2x80x9ccxe2x80x9d), followed by the fusion splicing between the high NA silicate fibers 68 and the pigtail fiber 67 (denoted by the symbol xe2x80x9caxe2x80x9d)
The first configuration of the conventional optical amplifier as shown in FIG. 21 includes many components such as the EDFs 44 and 51, the pumping light sources 43 and 49, and the optical combiners 42 and 48, which presents a problem of increasing the size and cost of the optical amplifier.
As for the second configuration of the conventional optical amplifier as shown in FIG. 22, since the gain equalizer 62 is installed outside the gain medium EDF, that is, on the output side of the signal light, a problem arises of reducing the optical output power by an amount of the loss of the gain equalizer 62.
Furthermore, since the configuration comprising the EDF 65a or 65b in connection with the wavelength independent reflector 66 as shown in FIG. 25A or 25B requires the fusion splicing made by butting the angled polished surfaces followed by bonding, it has a problem of increasing the number of optical components, and hence increasing the cost of components and assembly.
FIG. 46 is a block diagram showing a first configuration of a conventional Raman amplifier. The Raman amplifier comprises an optical fiber 161, a gain medium of the Raman amplification; a pumping light source 162; an optical combiner 163 for multiplexing the light of the pumping light source 162 with signal light; and an optical isolator 164 for preventing residual reflection light from entering the Raman amplifier.
The pumping light source 162 comprises fiber gratings (FGs) 166 for narrowing oscillation wavelengths of a plurality of laser diodes (LDs) 165; a wavelength selective combiner (WSC) 167 for combining a plurality of wavelengths of the pump light; and an optical isolator 168 for eliminating external residual reflection light (see, Y. Emori et al., Proc. OFC, PD19, 1999).
FIGS. 47A-47C are block diagrams showing application schemes of a conventional or present invention Raman amplifier: FIGS. 47A and 47B each show a scheme when the Raman amplifier is applied to an optical fiber communication system; and FIG. 47C shows a scheme when it is applied to measurement comprising a light source 173 and a measuring system 174. Furthermore, FIG. 47A shows a case of employing a distributed amplifier that utilizes a transmission fiber itself constituting the transmission path as an amplifying medium; and FIG. 47B shows a case used as a linear repeater, post-amplifier or preamplifier installed at a post- or pre-stage of the transmission fiber constituting the transmission path.
Although FIG. 46 shows a case of backward pumping in which the propagation direction of the pump light is opposite to that of the signal light, the following description is also applicable to forward pumping in which the two propagation directions are the same, or to bidirectional pumping.
The optical fiber 161 as shown in FIG. 46 corresponds to a transmission fiber 171 or Raman fiber 172. The transmission fiber 171 consists of a 1.3 xcexcm zero-dispersion single-mode fiber (SMF) or 1.5 xcexcm dispersion-shifted fiber (DSF). In the Raman fiber 172 as shown in FIGS. 47B and 47C, the composition and structure parameters of the optical fiber are set to increase the Raman gain coefficient. Generally speaking, as to the silicate fiber, the Raman gain coefficient increases with the GeO2 concentration in the fiber core or with the decrease in the mode diameter of propagation light (with an increase in the numerical aperture).
The length of the transmission fiber 171 is several tens of kilometer, whereas that of the Raman fiber 172 is a few kilometers. Thus, when using the transmission fiber 171, the signal light is amplified dispersedly in the transmission fiber, whereas when using the Raman fiber 172, the signal light is amplified concentratedly at a pre- or post-stage of the transmission fiber 171.
As described above, using the transmission fiber 171 as the optical fiber 161 can improve the optical noise characteristic as compared with using the Raman fiber 172 as the optical fiber 161 because the former amplifiers the signal light dispersedly. On the other hand, using the Raman fiber 172 as the optical fiber 161 has a characteristic of obviating restriction to the transmission fiber 171 as compared with using the transmission fiber 171 as the optical fiber 161 because the former amplifies the signal light concentratedly.
The pumping light source 162 includes, as its lasers, wavelength multiplexed laser diodes (LDs), or single wavelength or wavelength multiplexed fiber Raman lasers (see, K. Rottwitt et al., Proc. OFC, PD6, 1998 or E. M. Dianov et al., Electron. Lette., Vol.34, No.7, pp.669-670, 1998). The Raman amplifier with the structure as shown in FIG. 46 is highly reliable, highly stable and compact, constituting a more practical LDs than others. Generally, since the pumping efficiency of the Raman amplification using the optical fiber is low, the pump light power from a few hundred milliwatts to a few Watts is required. The pump light power of a single LD is usually a few hundred milliwatts. Accordingly, it is normal that the wavelength division multiplexing configuration of the pump light is employed as shown in FIG. 46. Although three LDs are used in this case, any number of LD or LDs can be employed.
As described above, to achieve the wavelength division multiplexing of the pump light, the pumping light source comprises the fiber gratings (FGs) for narrowing the oscillation wavelengths of the LDs; the wavelength selective combiner (WSC) for carrying out the wavelength division multiplexing of the pump light waves of different wavelengths; and the optical isolator for removing the external residual reflection light. Generally, the LDs are a Fabry-Perot LD, the transmission width of the FGs is less than about 1 nm and the spacing between adjacent wavelengths of the LDs is about 10 nm. The oscillation spectrum width of the LDs before passing through the FGs is about 10 nm. Thus, a configuration is proposed in which the spacing between adjacent wavelengths of the LDs are set greater than 10 nm to obviate the FGs (see, H. Masuda et al., Proc. ECOC, Post Dead-Line Paper, pp.73-76, 1997, for example). When the number of LDs is one, it is obvious that the wavelength selective combiner is unnecessary. Generally, the wavelength division multiplexing of the pump light waves is employed not only for increasing the total pump light power, but also for increasing the bandwidth of the Raman gain. The wavelength selective combiner consists of a Mach-Zehnder waveguide circuit (MZ-PLC) or an arrayed waveguide (AWG).
FIG. 48 is a block diagram showing a second configuration of the conventional Raman amplifier that carries out optical polarization division multiplexing of the pump light waves using polarization beam couplers (PBC) 173 to achieve higher total pump light power than the configuration as shown in FIG. 46.
FIG. 49 shows the transmittance spectra of three ports of the wavelength selective combiner 167, which illustrates the high transmittance at the pump light wavelengths.
FIG. 50 is a diagram showing a third configuration of the conventional Raman amplifier, which is disclosed in a document, E. M. Dianov et al., Electron. Lett., Vol.34, No.7, pp.669-670, 1998. The third configuration differs from the first and second configurations in that it carries out the pumping of the optical fiber using a single wavelength rather than using a plurality of wavelengths as in the first and second configurations. A high NA fiber 171 as shown in FIG. 50 is a kind of the optical fibers. FIG. 51 schematically illustrates the difference between a gain spectrum of the single wavelength pumping and that of the multiwavelength pumping. The multiwavelength pumping has wider bandwidth. This differs clearly from the case of the EDFA which employs the multiwavelength pumping to increase the pump light power. In the EDFA, the gain spectra are the same for both the single wavelength pumping and the multiwavelength pumping.
In the third configuration, a considerable amount of the pump light emitted from the high NA fiber 171 is reflected to be incident on the high NA fiber again, increasing its total pump light. As a result, the gain is increased because the Raman gain (internal value) in terms of dB is proportional to the total pump light power. The reflection of the pump light is carried out by guiding the pump light emitted from the high NA fiber to an input port of the 4-port fiber optical combiner and divider to divide it from the signal light, by causing the pump light emitted from the output port of the 4-port fiber optical combiner and divider to reflect off a fiber grating (FG) 175, and by returning the reflected pump light from the output port to the input port. In FIG. 50, the reference numeral 174 designates an optical isolator, 172 designates an optical combiner and 173 designates the optical combiner and divider.
As described above, the conventional Raman amplifier has low pumping efficiency, and hence requires high power pumping light source, which presents a problem of increasing the number of components of the pumping light source and its cost. In contrast, when the total pump light power of the pumping light source is limited, it presents a problem of being unable to provide a sufficient gain of the signal light.
Furthermore, in the third configuration, the pump light emerging from the high NA fiber passes through the 4-port fiber optical combiner and divider twice before returning to the high NA fiber, and hence undergoes a loss because of the insertion loss of the 4-port fiber optical combiner and divider. This presents a problem of being unable to neglect the insertion loss because it is no less than about 0.5 dB for a single passage, and 1 dB for twice passages. Moreover, the 4-port fiber optical combiner and divider has wavelength dependence of the insertion loss in the signal light wavelength region, and this presents a problem of providing an excessive loss for a wideband signal light.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide a small, inexpensive optical amplifier with a reduced number of components.
Another object of the present invention is to provide an optical amplifier capable of increasing its gain and pumping efficiency, and an optical fiber communication system using the optical amplifier.
To accomplish the foregoing problems, according to one aspect of the present invention, there is provided an optical amplifier comprising: a first optical circulator having a first port for receiving signal light, and a second port for emitting the signal light; a pumping light source for generating pump light; an optical combiner for combining the signal light emitted from the second port of the first optical circulator with the pump light fed from the pumping light source; an active optical fiber constituting a gain medium for receiving the signal light and pump light from the optical combiner, and for amplifying the signal light with being pumped by the pump light; and a second optical circulator having a first port for receiving the signal light passing through the active optical fiber, a second port for emitting the signal light, and a third port for receiving the signal light again to return it to the first port to be output from the first port, wherein the signal light emitted from the first port of the second optical circulator passes through the active optical fiber again in a direction opposite to a first incident direction, and the signal light passing through the active optical fiber in the opposite direction is incident on the second port of the first optical circulator via the optical combiner to be output from a third port of the first optical circulator.
This makes it possible to reduce the number of the components because the signal light passing through the first optical circulator and optical combiner and then through the active optical fiber is returned toward the active optical fiber through the second optical circulator to be incident again on the active optical fiber from the opposite direction.
According to another aspect of the present invention, there is provided an optical amplifier comprising: a first optical circulator having a first port for receiving signal light, and a second port for emitting the signal light; a pumping light source for generating pump light; an optical combiner for combining the signal light emitted from the second port of the first optical circulator with the pump light fed from the pumping light source; an active optical fiber constituting a gain medium for receiving the signal light and pump light from the optical combiner, and for amplifying the signal light with being pumped by the pump light; a gain equalizer on which the signal light and pump light passing through the active optical fiber are incident, the gain equalizer being antireflective for the signal light; and a reflector for reflecting the signal light emitted from the gain equalizer to be incident on the gain equalizer, wherein the signal light reflected by the reflector and passing through the gain equalizer again passes through the active optical fiber again in a direction opposite to a first incident direction, and the signal light passing through the active optical fiber in the opposite direction is incident on the second port of the first optical circulator via the optical combiner to be output from a third port of the first optical circulator.
This makes it possible to considerably reduce the number of components because the signal light passing through the optical circulator and optical combiner and then through the active optical fiber passes through the antireflective gain equalizer twice before and after reflected on the reflector to be incident on the active optical fiber again from the opposite direction, halving the transmission loss as compared with the conventional system because of passing through the antireflective gain equalizer twice.
According to still another object of the present invention, there is provided an optical amplifier comprising: a first optical circulator having a first port for receiving signal light, and a second port for emitting the signal light; a pumping light source for generating pump light; an optical combiner for combining the signal light emitted from the second port of the first optical circulator with the pump light fed from the pumping light source; an active optical fiber constituting a gain medium for receiving the signal light and pump light from the optical combiner, and for amplifying the signal light with being pumped by the pump light; and a reflector for reflecting the signal light passing through the active optical fiber such that the signal light travels through the active optical fiber in a direction opposite to a first incident direction, the reflector having a reflectivity depending on a wavelength of the signal light, and having a gain equalizing function, wherein the signal light reflected by the reflector after undergoing gain equalization by the reflector passes through the active optical fiber again in the direction opposite to the first incident direction, and the signal light passing through the active optical fiber in the opposite direction is incident on the second port of the first optical circulator via the optical combiner to be output from a third port of the first optical circulator.
This makes it possible to obviate the gain equalizer needed in the conventional system and to reduce the number of the components because the signal light passing through the optical circulator and optical combiner and then through the active optical fiber reflects off the reflector with the gain equalization function to be incident on the active optical fiber again from the opposite direction.
The reflector may comprise a fiber grating disposed in the active optical fiber for reflecting the signal light, and wherein the fiber grating carries out the gain equalization with a reflectivity independent of the signal light wavelength or a reflectivity dependent on the signal light wavelength.
This makes it possible to reduce the number of the components and the cost of the system because the reflector is composed of the fiber grating incorporated in the active optical fiber to reflect off the signal light.
The active optical fiber may consist of one of a rare-earth doped fiber and a Raman fiber.
This makes it possible to employ a rare-earth doped fiber or Raman fiber as the active optical fiber.
The second optical circulator may transmit at a low loss the pump light which is emitted from the pumping light source and passes through the active optical fiber, and launch the pump light into the active optical fiber again.
This makes it possible to reduce the number of the components and cost of the system because the second optical circulator passes the pump light, which is output from the pumping light source and passes through the active optical fiber, at the low loss to be incident on the active optical fiber again.
The reflector may comprise a mirror for reflecting at a high reflectivity the pump light which passes through the active optical fiber and the gain equalizer, and is incident on the mirror.
This makes it possible to prevent the reduction in the optical output power of the signal light which passes through the gain equalizer and is amplified by the active optical fiber because the reflector comprises the mirror for reflecting the pump light, which passes through the gain equalizer and is incident on the mirror, at high reflectivity.
The reflector with the wavelength dependent reflectivity may comprise a pump light fiber grating and a signal light fiber grating deposited in series for reflecting the pump light passing through the active optical fiber at a high reflectivity.
This makes it possible to reflect the pump light passing through the active optical fiber at the high reflectivity because the reflector with the wavelength dependent reflectivity comprises the fiber grating for the pump light and the fiber grating for the signal light.
The fiber grating disposed in the active optical fiber may comprise a pump light fiber grating and a signal light fiber grating deposited in series for reflecting the pump light passing through the active optical fiber at a high reflectivity.
This makes it possible to reflect the pump light passing through the active optical fiber at high reflectivity because the fiber grating incorporated in the active optical fiber comprises the pump light fiber grating and signal light fiber grating.
The optical amplifier may further comprise a second optical amplifier that is disposed at an input side of the first port of the first optical circulator on which the signal light is incident, that includes a second active optical fiber, and that has a sufficient gain for reducing a noise figure of the optical amplifier after the first optical circulator.
This makes it possible to positively reduce the noise figure of the optical amplifier after the optical circulator because the system comprises the short second active optical fiber installed before the optical amplifier, that is, at the input side of the optical circulator on which the signal light is incident, thereby providing the second optical amplifier with the sufficient amplification.
According to another aspect of the present invention, there is provided an optical amplifier comprising: an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber; an optical combiner for combining pump light fed from the pumping light source with input signal light; and a pump light reflector for reflecting the pump light passing through the optical fiber.
It corresponds to the embodiments as shown in FIGS. 26, 28 and 30, which is capable of increasing the total power of the input pump light because the pump light passing through the optical fiber without loss is reflected by the pump light reflector to be incident to the optical fiber again. Since the internal Raman gain (the gain in the optical fiber measured in terms of dB) is proportional to the total power of the input pump light, the internal Raman gain is considerably increased as compared with that of the conventional Raman amplifier, which improves the pumping efficiency of the Raman amplifier.
According to another aspect of the present invention, there is provided an optical amplifier comprising: an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber; an optical combiner and divider for combining and dividing the pump light from the pumping light source with and from input signal light; a signal light reflector for reflecting the signal light passing through the optical fiber; and an optical circulator for emitting from its two different ports the signal light reflected by the signal light reflector and the input signal light.
It corresponds to the embodiment as shown in FIG. 31, and has an advantage over the conventional configuration of being able to double the signal light gain so long as the Raman gain is not saturated by the signal light.
According to another aspect of the present invention, there is provided an optical amplifier comprising: an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber from an opposite direction to a propagation direction of signal light; a signal light reflector for reflecting the signal light passing through the optical fiber; and an optical circulator for receiving at its two different ports the signal light reflected by the signal light reflector and the input signal light, wherein the signal light reflector transmits the pump light fed from the pumping light source at a low loss.
It corresponds to the embodiment as shown in FIG. 33, and differs from the embodiment as shown in FIG. 31 in that it does not comprise the optical combiner and antireflection terminator. As to the improvement of the signal light gain, it offers an advantage over the conventional configuration of being able to double the signal light gain.
According to another aspect of the present invention, there is provided an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber from an opposite direction to a propagation direction of signal light; a signal light reflector for reflecting the signal light passing through the optical fiber; a pump light reflector for reflecting pump light passing through the optical fiber; and an optical circulator for receiving at its two different ports the signal light reflected by the signal light reflector and the input signal light, wherein the signal light reflector transmits the pump light fed from the pumping light source at a low loss, and the pump light reflector transmits the signal light at a low loss.
It corresponds to the embodiment as shown in FIG. 34, and has the advantages of both the embodiments as shown in FIGS. 26 and 33, thus markedly improving the signal light gain.
According to another aspect of the present invention, there is provided an optical amplifier comprising: an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber from a forward direction with respect to a propagation direction of signal light; an optical combiner and divider for combining and dividing pump light fed from the pumping light source with and from input signal light; a signal light reflector for reflecting the signal light passing through the optical fiber; a pump light reflector for reflecting pump light fed from the pumping light source and passing through the optical fiber; and an optical circulator for receiving at its two different ports the signal light reflected by the signal light reflector and the input signal light, wherein when the signal light reflector is disposed closer to the optical fiber than the pump light reflector is, the signal light reflector transmits the pump light fed from the pumping light source at a low loss, whereas when the pump light reflector is disposed closer to the optical fiber than the signal light reflector is, the pump light reflector transmits the signal light at a low loss.
It corresponds to the embodiment as shown in FIG. 35, and has the advantages of both the embodiments as shown in FIGS. 26 and 33, thus markedly improving the signal light gain as the embodiment as shown in FIG. 34.
The pumping light source may comprises a plurality of lasers with different wavelengths, and the pump light reflector may reflect all the pump light with different wavelengths fed from the lasers.
It corresponds to a combination of the embodiment as shown in FIG. 26 and one of the embodiments as shown in FIGS. 31, 33, 34 and 35.
Here, the pumping light source may comprise a laser diode with a gain independent of a polarization of propagated light, wherein the laser diode may have a front end surface undergone antireflection processing and a rear end surface undergone reflection processing, and wherein the pump light reflector may reflect the pump light with different wavelengths fed from the laser, and transmit the signal light at a low loss.
The pumping light source may comprise a plurality of laser diodes for oscillating light waves of different wavelengths with a gain independent of polarization of propagated light, and a wavelength selective optical combiner and divider for combining the plurality of pump light waves fed from the plurality of laser diodes, wherein the laser diodes may each have a front end surface undergone antireflection processing and a rear end surface undergone reflection processing, and wherein the pump light reflector may reflect all the pump light waves with different wavelengths fed from the laser diodes, and transmit the signal light at a low loss.
The pumping light source may comprise a laser diode with its front end surface undergone an antireflection processing and its rear end surface undergone a high reflection processing, wherein the pump light reflector may reflect the pump light with different wavelengths fed from the laser diode, and transmit the signal light at a low loss, and wherein optical components transmitting the pump light from the pumping light source may each consist of a polarization maintaining optical component for the pump light.
The pumping light source may comprise a plurality of laser diodes for oscillating light waves of different wavelengths, and a wavelength selective optical combiner and divider for combining the plurality of pump light waves fed from the plurality of laser diodes, wherein the laser diodes may each have a front end surface undergone antireflection processing and a rear end surface undergone reflection processing, wherein the pump light reflector may reflect all the pump light waves with different wavelengths fed from the laser diodes, and transmit the signal light at a low loss, and wherein optical components transmitting the pump light from the pumping light source may each consist of a polarization maintaining optical component for the pump light.
These aspects of the present invention correspond to the combination of the embodiment as shown in FIG. 28 and one of the embodiments as shown in FIGS. 31, 33, 34 and 35.
The pumping light source may comprise a pair of laser diodes for oscillating light waves of a same wavelength, and a polarization beam combiner and divider for combining two pump light waves with different polarization from two laser diodes, wherein the laser diodes may each have a front end surface undergone antireflection processing and a rear end surface undergone reflection processing, and wherein the pump light reflector may reflect the plurality of the pump light waves with different wavelengths fed from the laser diodes, and transmit the signal light at a low loss.
The pumping light source may comprise a plurality of pairs of laser diodes, each pair including two laser diodes for oscillating light waves of a same wavelength; a natural number of polarization beam combiner and dividers each for combining pump light waves from two laser diodes in each pair, and a wavelength selective optical combiner and divider for combining the combined pump light waves output from the polarization beam combiner and dividers, wherein the laser diodes may each have a front end surface undergone antireflection processing and a rear end surface undergone reflection processing.
These aspects of the present invention correspond to a combination of the embodiment as shown in FIG. 30 and one of the embodiments as shown in FIGS. 31, 33, 34 and 35.
According to another aspect of the present invention, there is provided an optical fiber communication system for transmitting signal light via an optical fiber using an optical amplifier, the optical amplifier comprising: an optical fiber operating as a gain medium of a Raman amplifier; a pumping light source for pumping the optical fiber; an optical combiner for combining pump light fed from the pumping light source with input signal light; and a pump light reflector for reflecting the pump light passing through the optical fiber, wherein the optical fiber is a transmission optical fiber for amplifying the optical signal dispersedly.
It corresponds to the optical fiber communication system utilizing one of the Raman amplifiers as shown in FIGS. 26, 28 and 30, which accept either a plurality of waves undergone wavelength division multiplexing or a single wavelength passing through time division multiplexing. In a field associated with the optical fiber communication system such as measurement, the signal light may consist of a plurality of wavelengths or a single wavelength.
According to one aspect of the present invention, the number of components and the cost of the components and assembly can be reduced because it is configured such that the signal light passing through the first optical circulator and optical combiner and then through the active optical fiber is returned via the second optical circulator to be incident on the active optical fiber again from the opposite direction.
According to another aspect of the present invention, the number of components and the cost of the components and assembly can be reduced because it is configured such that the signal light passing through the optical circulator and optical combiner and then through the active optical fiber passes through the antireflective gain equalizer twice before and after reflected by the reflector, and is incident on the active optical fiber again from the opposite direction. In addition, since the signal light passes through the antireflective gain equalizer twice, the transmission loss is reduced as compared with that of the conventional system.
According to another aspect of the present invention, the number of components and the cost of the components and assembly can be reduced because it is configured such that the signal light passing through the optical circulator and optical combiner and then through the active optical fiber is reflected by the reflector with the function of the gain equalization, and is incident on the active optical fiber again from the opposite direction. In particular, since it obviates the gain equalizer required by the conventional system, it can further reduce the number of components.
According to another aspect of the present invention, the number of components and the cost can be reduced because its reflector consists of the fiber grating incorporated in the active optical fiber for reflecting the signal light.
According to another aspect of the present invention, the number of components and the cost can be reduced because the second optical circulator transmits the pump light, which is emitted from the pumping light source and passes through the active optical fiber, at the low loss, and makes it incident on the active optical fiber again.
According to another aspect of the present invention, the number of components and the cost can be reduced because the reflector comprises the mirror for reflecting at the high reflectivity the pump light which passes through the active optical fiber and is incident on the mirror after passing through the gain equalizer. In addition, since the signal light is amplified again by the active optical fiber after passing through the gain equalizer, the reduction in the optical output power can be prevented.
According to another aspect of the present invention, the number of components and the cost can be reduced because the reflector with wavelength dependent reflectivity comprises the pump light fiber grating and the signal light fiber grating, which also makes it possible to reflect the pump light passing through the active optical fiber at the high reflectivity.
According to another aspect of the present invention, the number of components and the cost can be reduced because the fiber grating incorporated in the active optical fiber comprises the pump light fiber grating and signal light fiber grating, which also makes it possible to reflect the pump light passing through the active optical fiber at the high reflectivity.
According to another aspect of the present invention, the noise figure of the optical amplifier after the optical circulator can be positively reduced because it comprises not only the short second active optical fiber before the optical amplifier, that is, at the input side of the optical circulator on which the signal light is incident, but also the second optical amplifier with the sufficient gain.
According to another aspect of the present invention, the Raman amplifier comprises an optical fiber as the gain medium of the Raman amplifier, a pumping light source for pumping the optical fiber, an optical combiner for coupling the pump light fed from the pumping light source with the signal light, and a pump light reflector for reflecting the pump light passing through the optical fiber, and reflects the pump light passing through the optical fiber without loss by the pump light reflector to be incident on the optical fiber again, which makes it possible to increase the total power of the input pump light. On the other hand, since the internal Raman gain is proportional to the total power of the input pump light, the internal Raman gain is increased considerably as compared with that of the conventional Raman amplifier, offering an advantage of being able to improve the pumping efficiency of the Raman amplifier.
Using the embodiments of the optical amplifier in accordance with the present invention makes it possible to construct an optical fiber communication system that can accept either the signal light including a plurality of wavelengths passing through the wavelength division multiplexing, or the signal light consisting of a single wavelength passing through the time division multiplexing.